The present invention relates generally to micro-electromechanical devices and, more particularly, to micro-electromechanical thermal actuators such as the type used in ink jet devices and other liquid drop emitters.
Micro-electro mechanical systems (MEMS) are a relatively recent development. Such MEMS are being used as alternatives to conventional electro-mechanical devices as actuators, valves, and positioners. Micro-electromechanical devices are potentially low cost, due to use of microelectronic fabrication techniques. Novel applications are also being discovered due to the small size scale of MEMS devices.
Many potential applications of MEMS technology utilize thermal actuation to provide the motion needed in such devices. For example, many actuators, valves and positioners use thermal actuators for movement. In some applications the movement required is pulsed. For example, rapid displacement from a first position to a second, followed by restoration of the actuator to the first position, might be used to generate pressure pulses in a fluid or to advance a mechanism one unit of distance or rotation per actuation pulse. Drop-on-demand liquid drop emitters use discrete pressure pulses to eject discrete amounts of liquid from a nozzle.
Drop-on-demand (DOD) liquid emission devices have been known as ink printing devices in ink jet printing systems for many years. Early devices were based on piezoelectric actuators such as are disclosed by Kyser et al., in U.S. Pat. No. 3,946,398 and Stemme in U.S. Pat. No. 3,747,120. A currently popular form of ink jet printing, thermal ink jet (or xe2x80x9cbubble jetxe2x80x9d), uses electroresistive heaters to generate vapor bubbles which cause drop emission, as is discussed by Hara et al., in U.S. Pat. No. 4,296,421.
Electroresistive heater actuators have manufacturing cost advantages over piezoelectric actuators because they can be fabricated using well developed microelectronic processes. On the other hand, the thermal ink jet drop ejection mechanism requires the ink to have a vaporizable component, and locally raises ink temperatures well above the boiling point of this component. This temperature exposure places severe limits on the formulation of inks and other liquids that may be reliably emitted by thermal ink jet devices. Piezoelectrically actuated devices do not impose such severe limitations on the liquids that can be jetted because the liquid is mechanically pressurized.
The availability, cost, and technical performance improvements that have been realized by ink jet device suppliers have also engendered interest in the devices for other applications requiring micro-metering of liquids. These new applications include dispensing specialized chemicals for micro-analytic chemistry as disclosed by Pease et al., in U.S. Pat. No. 5,599,695; dispensing coating materials for electronic device manufacturing as disclosed by Naka et al., in U.S. Pat. No. 5,902,648; and for dispensing microdrops for medical inhalation therapy as disclosed by Psaros et al., in U.S. Pat. No. 5,771,882. Devices and methods capable of emitting, on demand, micron-sized drops of a broad range of liquids are needed for highest quality image printing, but also for emerging applications where liquid dispensing requires mono-dispersion of ultra small drops, accurate placement and timing, and minute increments.
A low cost approach to micro drop emission is needed which can be used with a broad range of liquid formulations. Apparatus and methods are needed which combines the advantages of microelectronic fabrication used for thermal ink jet with the liquid composition latitude available to piezo-electro-mechanical devices.
A DOD ink jet device which uses a thermo-mechanical actuator was disclosed by T. Kitahara in JP 2,030,543, filed Jul. 21, 1988. The actuator is configured as a bi-layer cantilever moveable within an ink jet chamber. The beam is heated by a resistor causing it to bend due to a mismatch in thermal expansion of the layers. The free end of the beam moves to pressurize the ink at the nozzle causing drop emission. Recently, disclosures of a similar thermo-mechanical DOD ink jet configuration have been made by K. Silverbrook in U.S. Pat. Nos. 6,067,797; 6,087,638; 6,239,821 and 6,243,113. Methods of manufacturing thermo-mechanical ink jet devices using microelectronic processes have been disclosed by K. Silverbrook in U.S. Pat. Nos. 6,180,427; 6,254,793 and 6,274,056.
Thermo-mechanically actuated drop emitters employing a cantilevered element are promising as low cost devices which can be mass produced using microelectronic materials and equipment and which allow operation with liquids that would be unreliable in a thermal ink jet device. However, the design and operation of cantilever style thermal actuators and drop emitters requires careful attention to locations of potentially excessive heat, xe2x80x9chot spotsxe2x80x9d, especially any within the cantilevered element which may be adjacent to the working liquid. When the cantilever is deflected by supplying electrical energy pulses to an on-board resistive heater, the pulse current is, most conveniently, directed on and off the moveable (deflectable) structure where the cantilever is anchored to a base element. Thus the current reverses direction at some locations on the cantilevered element. The locations of current directional change may be places of higher current density and power density, resulting in hot spots.
Hot spots are locations of several potential reliability problems, including loss of resistivity or catastrophic melting of resistive materials, electromigration of ions changing mechanical properties, delamination of adjacent layers, cracking and crazing of protective materials, and accelerated chemical interactions with components the working liquid. An additional potential problem for a thermo-mechanically activated drop emitter is the production of vapor bubbles in the working liquid immediately adjacent a hot spot. This latter phenomenon is purposefully employed in thermal ink jet devices to provide pressure pulses sufficient to eject ink drops. However, such vapor bubble formation is undesirable in a thermo-mechanically actuated drop emitter because it causes anomalous, erratic changes in drop emission timing, volume, and velocity. Also bubble formation may be accompanied by highly aggressive bubble collapse damage and a build-up of degraded components of the working liquid on the cantilevered element.
Designs for thermal ink jet bubble forming heater resistors which reduce current crowding have been disclosed by Giere, et al., in U. S. Pat. No. 6,280,019; by Cleland in U.S. Pat. Nos. 6,123,419 and 6,290,336; and by Prasad, et al., in U.S. Pat. No. 6,309,052. Thermal ink jet physical processes, device component configurations and design constraints, addressed by these disclosures, have substantial technical differences from a cantilevered element thermo-mechanical actuator and drop emitter. The thermal ink jet device must generate vapor bubbles to eject drops, a thermo-mechanical drop emitter preferably avoids vapor bubble formation.
Configurations and methods of operation for cantilevered element thermal actuators are needed which can be operated at high repetition frequencies and with maximum force of actuation, while avoiding locations of extreme temperature or generating vapor bubbles.
It is therefore an object of the present invention to provide a thermo-mechanical actuator which does not have locations which reach excessive, debilitating, temperatures, and which can be operated at high repetition frequencies and for millions of cycles of use without failure.
It is also an object of the present invention to provide a liquid drop emitter which is actuated by a thermo-mechanical actuator which does not have locations which reach temperatures that cause vapor bubble formation in the working liquid.
The foregoing and numerous other features, objects and advantages of the present invention will become readily apparent upon a review of the detailed description, claims and drawings set forth herein. These features, objects and advantages are accomplished by constructing a thermal actuator for a micro-electromechanical device comprising a base element and a cantilevered element extending from the base element and normally residing at a first position before activation. The cantilevered element includes a first layer constructed of an electrically resistive material, such as titanium aluminide, patterned to have a first resistor segment and a second resistor segment each extending from the base element. The cantilevered element also includes a coupling segment patterned in the electrically resistive material, or a coupling device formed in an electrically active material, that conducts electrical current serially between the first and second resistor segments. A second layer constructed of a dielectric material having a low coefficient of thermal expansion is attached to the first layer. A first electrode connected to the first resistor segment and a second electrode connected to the second resistor segment are provided to apply an electrical voltage pulse between the first and second electrodes thereby causing an activation power density in the first and second resistor segments and a power density maximum within the coupling segment or device, resulting in a deflection of the cantilevered element to a second position and wherein the power density maximum is less than four times the activation power density. The coupling segment may also be formed in a portion of the first layer wherein the electrically resistive material is thick or has been modified to have a substantially higher conductivity.
The present invention is particularly useful as a thermal actuator for liquid drop emitters used as printheads for DOD ink jet printing. In this preferred embodiment the thermal actuator resides in a liquid-filled chamber that includes a nozzle for ejecting liquid. The thermal actuator includes a cantilevered element extending from a wall of the chamber and a free end residing in a first position proximate to the nozzle. Application of a heat pulse to the cantilevered element causes deflection of the free end forcing liquid from the nozzle.